[AnotherGoodName]

Used makes sense for getting a feeling for pure performance (smaller the better as its likely to be in memory).

Available makes sense for knowing when things will just plain break (reaching 0 = write failure for a DB).

>Every few months we get an alert from our database monitoring to warn us that we are about to run out of space.

In this case they were avoiding their DB server breaking. They didn't do this for performance reasons.